1. Field of the Invention
The present invention relates to an analog/digital converter (A/D converter) including an integration circuit having a switched capacitor circuit in an input section of an analog signal.
2. Description of the Background Art
A technique to convert the analog signal to a 1-bit digital signal by delta sigma (ΔΣ) modulation is widely known in the field of the analog/digital converter (A/D converter). A delta sigma modulation circuit includes an integration circuit and a quantizer, and a switched capacitor integration circuit is conventionally frequently used as the integration circuit (refer to “Applied Technology of Signal Processing by OP Amplifier, All About OP Amplifier, Vol. 2”, Analog Devices, CQ Publishing, Feb. 1, 2005, p. 115, for example).
In the A/D converter in which the switched capacitor integration circuit is used in an input section of the analog signal, a driver circuit for inputting the analog signal to the A/D converter is connected to the switched capacitor circuit. In such a case, a problem occurs that a spike-like noise (referred to as a “kickback noise”) is generated when rapidly charging a sampling capacitor in the switched capacitor circuit, and this is superimposed on an analog signal waveform (detail thereof will be described later). Then, accuracy of the input analog signal is destroyed, so that deterioration in accuracy of A/D conversion occurs.
As measures for solving the problem, it is considered to improve drive ability of the above-described driver circuit and to provide a circuit (refer to a circuit “LPF in FIG. 8) including a capacitative element for supplementing transient outflow and inflow of an electric charge and resistor to prevent an amplifier output of a previous stage from oscillating due to the capacitative element in the input section of the A/D converter, as suggested in “Applied Technology of Signal Processing by OP Amplifier, All About OP Amplifier, Vol. 2”, Analog Devices, CQ Publishing, Feb. 1, 2005, p. 115. However, any of the measures is not preferable for increasing a forming area of the circuit. Meanwhile, in this specification, the circuit including the above-described capacitative element (C) and resistor element (R) is referred to as an “RC low-pass filter” for convenience of description.